Low-pressure product drying



Dec. 9, 1952 H. o. MQMAHON 2,620,573

LOW-PRESSURE PRODUCT DRYING I Filed Jan. 28, 1948 2 SHEETS-SHEET 1 Figollo [TH W1 ITH HOWARD o. MMAHON INVENTOR.

1386- 1952 H. o. MCMAHON LOW-PRESSURE PRODUCT DRYING 2 SHEETS-SHEET 2 Filed Jan. 28, 1948 1:2: =5: 25555-5: IE. 3 I all...

HOWARD O. M MAHON I INVENTOR. y%7% ATTORNEY Patented Dec. 9, 19 52 Low-PREssnRm PRODUCT DRING-- HbwzirdOYlVitiMiihom Lexington, Mass ,assig n'or to Art-hur -Dr liittl'eg 1m, Gambrid'ga -Massr, a

corporatiomofi Massachusetts Application'Jamlamy 128, 19,48;- Serial-No.=4,920

of?t1ie".ma"terial,heingidriedZto l retain its original? 1 6 properties suchas' vitamins" andflavc'r; in thelcase of "fo'o'dpmductsfaswelf'aslthe" ease with. which" it maybe reconstituted: uponirth'e; addition of" water:

Florffreezet-dryirigg to projceedji it is necessary that the"total"pressure"iriztl'ievacuumi cha'mber, i: e." theipressurewf thepwaterl vaporliberatew from. tl'i'e'materiafand the. residual gas. pressure v. after'evacuatiomhetless *tli'an'thattcorresponding 7 dehydration: Inthecase of purewaterymaintee; nance of a total pressure of less. than iffi mm'. absolute:- isrequiredto"produ'ceireezing; andfor material-shavmg a lower freezing pointthan" water accorresporrdingli lowerpres'sm'e" must'bf'e' maixrtaine dc i -Heretofbre-=attempts have beem made" tot dry materials from the frozen state by continuousri meansfor' i'nst'ance while thamateri'a'l is" on a moving conve$ er-= such" as -a drum: an" endless" beltor'tiie lilie. fi thy-material isintro duced 'ihto the vacuuxmwchamldeciir'unfrozen-condition, ap

plyingdt to tlienonveyer means hasinvolved-con sidrabl difii'cuities =-principa1ly on account of the rapidi ty with-which the water content-oi the material -fieczesand=- becomes unmanageabler v eitherclogging theorifice of "the introduction means or adhering t'oe surfaces in i the chamber wheresuch iadh erenca is undesirable? Direct spraying of the liquid material on-sueli 40 a conveyor, in whichthespray-characteristics of tliezliquid; are. controlled' so thata liquid filr'n 'is formediomtheiconveyer prior-tofreezing; appears tos be'ttheamost sipracticalsmethodi of application heretofore-:pnoposed; but I imtliismethod there is an; undesirahle loss s offiproduct occasioned b'y spindrift ;whiclizcausesisome; of; theesprayto'zveerw to.-=t'he sidewandcmisszitsrobcect camizzazlso because somezot the.:liquidzmateriaizbouncesmr spatter-s;

off thamonveyer; and iodges-aoniothei' surfaces; 1 a 5 Accordineiztow thee presentc'invention, the: ma teriarktoube: driediis: introducedz'inttr-tlia vacuum 1 ch-amberr-fasi-aziliquids andzis appliedttmth s al veyeii in: thiszform. This-.is accomplished by -en closing the portion of the conveyer on the" material is initially applied; andmaintainin'gi withinithe enclosure a vacuum which is sufii cient-v toproduce considerable cooling of the materi'alby evaporation of some of'its water content. the vacuum. heing rinsufiici'ent' however to cause freezing of"the;m ateria1. Thewatervapor evolved from the, material" in the enclosure is' removed at a rate'such th'attan' equilibrium pres'- sure condition is maintained 'withirrtlie enclosure during theiconstantapplicatiom of the material: As s00'n"as the" conveyor leave's'th'e' enclosure; the; freezinggoffthe torthe' movinghconveyer' means:

liquid, which has beendi's'pjose'd as a film thereon may, occur," and"drying. iof material upon the ape pliicatio'n' of heat'and the maintenance of vacuum.

conditions proceeds in an effiicient manner.

The invention may beicarriewout" byvarious means; several" of whichwiilbe hereinafter described :indetail'. It will benotedhowever' that th'e objects "of theinvention areaccomplis'hed" without limitation vto specific-means; saidobjects including; v

The provision' of 'applic'atonmeans forfacontinuousfreeze 'di'ierinrwhichthedryingmaypro Theeliminaticn'o'fifthe'lossof ultimate prod= uct because of the: lodging of th'e material on undesira-loie'v surfaces and"hecomirrgfcontaminated or'umec'overableuw apracticalfmanner'.

Freedom frominterruptionrin operatiomof the dryingapparatushecause ofcloggingi'cfjthefma V terial" introduction" means-or irregular" perfcrn rance'ofthe'same:

Specific-embodimentsof the inventioniaretillus trated in' the' accompanying'idrawings whereini Fi'g: l'isa diagrammatic viewoftl'ieside elevation" of apparatus; suitable for carrying outrtl'ia present invention; j

Fig. 2 is an enlarged side elevation'ofthejapa- Fig; 3 is' a section taken on the: line 3 3," of; Fig? 2; and" Figi Pisa side elevation of "a modifiedgifornif.

ofappara-tusi The materials that may be'drie'din-"thaman;

ner-hereinafter described covera widerange of? substances such as fruit juices; milk; coffee "ex= tract; eggs and" similar food-productwas well; as" sera and otl iei 'laic'ilogicallyv active substances. I On:

"account of=the different characteristics"ofithese; V different substances' aznd variationsfin" their. solid;

contents;- edryine itechniq ies totb'e employed o -"b the a e?for allfplOdllctsjqlduttlie princi'pl'es toheiobs'erved'inregardtofthepresentt invention maybe considered th'e samet Referring to Fig. 1, the vacuum chamber II is sufficiently large to accommodate an endless belt I2 trained over pulleys I3 and I4 disposed adjacent each end of the chamber. The drying portion of the belt I2 rides on heated platens I5, the temperature of which is carefully controlled in relation to the speed of the belt so that when the belt reaches the pulley I4 the product thereon will be thoroughly dried and can be removed by the doctor blade IS. The platens I5 may be heated in any suitable manner, for instance by circulating heated water therethrough, suitable pipes l5 conducting such water to and. from the platens.

The vacuum chamber is initially evacuated. by a vacuum pump II, the pipe I8 which leads to the vacuum pump communicating with the vacuum chamber I I through a portion of the latter in which there is arranged a contact condenser I9 for the water vapor given off by the material. The contact condenser I9 which may be conveniently disposed directly beneath the belt conveyer may consist of a header pipe 20 from which a. spray of a concentrated refrigerated brine falls on the gas being drawn to the vacuum pump H. The falling brine is collected in a pan 2| and contains the condensed water vapor given off from the material. The now somewhat diluted brine is pumped from the chamber by the pump 22, and may be reconstituted (i. e. re-concentrated and re-refrigerated) prior to its reintroduction through the pipe 23 which leads to the header 20. The contact condenser forms no part of this invention, but must utilize a cooling medium that under the conditions existing in the vacuum chamber will not yield water apor thereto.

The rate of drying of the material on the endless belt may be augmented by the provision of radiant heating means 24 disposed above the belt and opposite to the platens.

The material to be dried is introduced in liquid form through the pipe 25 which enters the enclosure 26. This enclosure which is best illustrated in Figs. 2 and 3, is disposed adjacent the underside of the belt beneath one ofthe end pulleys of the conveyer and is of a width substantially the same as the belt I2. Rubber seals 21 are arranged to extend from all but one edge of the perimetral portions of the enclosure and by contacting the belt II they seal the enclosure from the rest of the inside of the vacuum chamber. The edge of the enclosure which is not provided with rubber seal has a series of orifices 28 of progressively smaller clearance with.

respect to the belt as it passes over the head pulley I3. Between these orifices are surge chambers 29 the purpose of which will be hereinafter described.

The liquid material that it is intended to dry is permitted to collect in the bottom of the enclosure until it reaches the level indicated by the dotted line shown in Figs. 1 and 2. A .roller 30 preferably having a rubber coating dips into the liquid in the enclosure and on rotation applies it to the outer surface of the conveyer belt. The roller 30 of course is journalled in suitable bearings 3I mounted in the chamber and is preferably power driven as by the gears 32 and 33 which derive their power from the shaft 34 on which the head pulley I3 is mounted. The portions of the shaft on which the roller is mounted, which extend through the sides of the enclosure, are provided with rotary seals 35 as illustrated in Fig. 3. A sight glass 36 on one side of the en- 4 closure permits observation of the level of the liquid within the enclosure and the control of the rate of feed of incoming liquid to maintain the same.

At the start of operations the vacuum chamber II is evacuated to the desired ultimate operating pressure. Because of the communication between the enclosure 26 and the vacuum chamber adjacent the outer edge of the belt, the enclosure will be simultaneously evacuated to the same extent. Next the material to be dried is fed through the pipe 25 in the enclosure until the desired level thereof is obtained. The roller applicator 30 is then set in operation and as the belt I2 moves, the roller supplies a film of liquid material to the belt which progresses past the end pulley I3 and over the heated zone between the platens I5 and the source of radiant heat 24.

Freezing of the material which occurs as the belt emerges from the last orifice 28 confining the opening to the enclosure is prevented within the enclosure by the evolution of water vapor in the enclosure caused by the low pressure prevailing in the enclosure at the time the material is first introduced in the enclosure. This water vapor escapes from the enclosure through the series of orifices previously described but its rate of escape is controlled by the shape of the metering orifices so that an equilibrium pressure condition is maintained within the enclosure which is just slightly above that corresponding to the pressure at which the material would otherwise freeze.

As previously stated, the actual pressure within the vacuum chamber and that within the enclosure will vary with different materials and different operating conditions. It has been found that a pressure within the enclosure of the order of 6 mm. Hg absolute is a suitable and practical pressure to maintain where the pressure within the vacuum chamber is of the order of one mm. Hg absolute.

The various orifices 28 which are arranged in series provide a rate of fiow of vapor from the interior of the enclosure such that the necessary pressure drop is obtained; the actual pressure drop which the metering orifices efiect will vary with the temperature of the material bein introduced, its latent heat of vaporization and the width of the slots which the orifices provide. The clearances must be relatively small, for the vacuum is so high that the velocity through the orifices is exceedingly great, and hence a pressure drop is not easily obtained. The pockets 29 which are provided between the orifices constitute chambers in which turbulence of the escaping gas may be created, thereby building up pressure in back of each of the orifices, and each of the orifices will therefore create a part of the total pressure drop.

In Fig. 4 a modified form of apparatus is disclosed in which instead of applying the material to the belt by a roller, one or more spray nozzles 40 are arranged to extend through the enclosure 4| and spray the material introduced through the pipe 42 directly on the moving belt I2 of the belt conveyer. Whatever bounces off the belt or impinges on the side of the enclosure 4| fiows on the side of the enclosure and hence onto the belt since the material is still in liquid form. All of the other parts of the apparatus shown in Fig. 4 are the same as those shown in Fig. 1 and a description thereof need not be repeated, like numbers on $16 drawings in both views referring to like par ReferringitagainitogEig, 1, therfinally dried product is a powder which may contain as. little ,as 1%.,moisture, and .is .removed by the. .doetor anaemic be collected 'ina liopper Ifidisposed 'rieatli'th doctor b'l'ade', then-lfpassing 'th "ough ;ser es;r,1ceks'4sbr aktypelwell knows; Lethe r-tI'ZTIZThe p'r'o'duct' maygber ijremoved' fronifglthe "amber'l without intrferingarvith the' vacuum cdnditionsgexisting inthelchainber. iilhusgdry- "ing'may proceed in a continuous manner and a high output .of dried product is-obtained.

The belt upon which the material is dried may be of-,any, suitable material; either organicior in- *"organic.'*Thus' it'may'consist*of'nylon or" a'suitably plasticized and heat-and-cold resistant plastic, or it may be a metal such as stainless steel;-Monel metal orsaluminumi 'I I-iabrine emcployed as-"awcontact condensermay be a cenc'entrated lithium chloridefsolution Automaticlevel =devices may be'-errlployedto-maintain -the *liq-uid ber 26 --at the desired-point; thus v V 'ustin'gthe lfeedr f =lSu'itialile ma- -nometerstodndi atethepressurejin the charnbers anti -2e @may 'be provided to eriabl moreiaccurat'e' observation "of the operation;and steamy positioned thermometers can be employed for a similar purpose.

Difierent modifications within the skill of those conversant with this art are to be construed as within the purview of the invention.

What is claimed is:

1. Apparatus for drying liquid materials comprising a vacuum chamber, moving conveyingmeans disposed within said, chamber, means for evacuating said vacuum chamber to a pressure at which auto-refrigerant freezing of the material occurs, means for supplying heat to material on said conveying-means to cause drying thereof, an enclosure confining a portion of the conveying-means, means for applying material to the conveying-means within said enclosure, said enclosure communicating with the vacuum chamber by a series of restricted openings of progressively smaller clearance through which the material applied on the conveying-means may pass together with suificient water vapor evolved during the application of the material to the conveying-means to maintain within the enclosure a substantially constant, vapor pressure higher than that at which auto-refrigerant freezing of the material occurs in the evacuated chamber, and means for removing from the vacuum chamber the water vapor given off by the material as it dries.

2. In apparatus of the class described for freeze-drying liquid materials in a highly evacuated chamber wherein the material is dried by auto-refrigerant freezing on moving conveyermeans arranged therein, the improvement which comprises the inclusion of means for applying the material to be dried as a liquid film on said conveying-means, and an enclosure surrounding said material application means, said enclosure having a series of metering orifices afiording communication between the enclosure and the chamber through which gas may be withdrawn during the evacuation of the chamber and through which water vapor evolved from the material in the enclosure may escape at a controlled rate during the application of material to the enclosure, said communicating orifices operating in series to maintain within the enclosure a substantially constant pressure greater than that in the chamber by an amount sufiicient to prevent freezing of the material in the enclosure.

3. In apparatus for freeze-drying liquid marterialsuin. atr highly zlev acuatedrchamberrmherein iiiithe material ?is.:dried on;moy ing:conveyor -imeans "arranged therein: the :improvement' ,whi'chucomprises means for applyingrthematerial to-bedried 5; as ...a1film..on ssaid conveyore-means ztam ie'nclosure l .:surroundingsaid;material applicatiommeans and I atsportion of: said lCOIWBYOEZaIIdE :commnnicating =means between; the. enclosure Elldil'llfi chamber ::for maintaining:withintsaidmnclosuneia'meduhed 1gipressurezgreaterrthan that within thbilminblilitO 1: :preyent freezing tofthezmateriali o'nzthe: ccmvieyerz-zsvacuum:existingximthe rliamberasaidrammunicmeans until:the materialcisi'subjedted'storthei'itrigh cating 'smeanszlinclnding. iairsferies ofinrificesrar- 1 .rangeda; tozizpro'duce .the: tdesiredri pressurzzdrop B-apartby surge-.itchambersiiimwhichiftherescaping water vayor may-:sbeccme rturbulentsrthereby building:upxpressure-insback of each orifice.

5. In a freeze-drying apparatus, a housing constituting a vacuum chamber, a conveyer arranged in said housing to convey materialbeing dried, evacuating apparatus,operatively, connected and arranged to exhaust said vacuum chamber so that the residual gas pressure and the pressure of water vapor liberated from the material is less than the pressure corresponding to the freezing point of the material and is of the order of one mm. Hg absolute, an applicator adapted and arranged to apply to said conveyor a film'of liquid material to be dried, and an enclosure enclosing said applicator and the contiguous portion of said conveyer, said enclosure presenting a series of metering orifices through which said conveyer conveys the film of liquid material sequentially from said enclosure into said vacuum chamber for freezing and drying, said series of metering orifices being of such vapor passing capacity as to permit the escape of vapor evaporated from said material in said enclosure while maintaining within said enclosure a vapor pressure substantially higher than that of the chamber and of the order of 6 mm. Hg absolute, whereby liquid material within said enclosure is evaporated while maintained at a temperature above its freezing temperature to facilitate its application to said conveyer for movement thereon into said freezing and drying vacuum chamber.

6. In a freeze-drying apparatus, a housing constituting a vacuum chamber, a conveyer arranged to operate within said housing for conveying material being dried, evacuating apparatus operatively connected and arranged to establish Within said housing a vacuum sufficiently high to eifect auto-refrigerant freezing of the material to be dried, an applicator adapted and arranged to apply to said conveyer a film of liquid material to be dried, and an enclosure enclosing said applicator and the contiguous portion of said conveyer, said enclosure presenting a series of metering orifices with surge chambers between them through which said conveyer conveys the film of liquid material from said enclosure into said vacuum chamber for freezing and drying, said series of metering orifices and intervening surge chambers being of such vapor passing cacapity as to permit the escape of vapor evaporated from said material in said enclosure while maintaining within said enclosure a vapor pressure higher than that at which auto-refrigerant freezing of the material occurs, whereby liquid material within said enclosure is maintained at a temperature above its freezing temperature to facilitate its application to said conveyer 'for movement thereon into said vacuum chamber as a liquid film prior to freezing therein. V

7. The method of freeze-drying material on a moving conveyer that comprises, subjecting liquid material to be dried to a vacuum not sufiiciently great to efiect auto-refrigerant freezing thereof the vacuum being of the order of 6 mm. Hg absolute to effect a partial drying action without freezing the material, applying the liquid material to the moving conveyer in the form of a thin film while subjected to the non-freezing vacuum, then subjecting the film of material as it advances on the conveyor to successive proto freeze-dry it, andthen removing-the dried material from the conveyer.

HOWARD O. MCMAHON.

REFERENCES CITED 7 V The following references are of record in the file of this patent:

8 UNITED STATES PAYENTS Number Name 7 Date 1,200,116 Johnson et al Oct. 3, 1916 2,087,788 Thal July 20, 1937 2,100,151 Tietz Nov. 23, 1937 2,396,561 Flosdorf Mar. 12, 1946 2,400,748 Flosdorf May 21, 1946 2,411,152 Folsom Nov. 19, 1946 2,528,476 Roos et al Oct. 31, 1950 FOREIGN PATENTS Number Country Date 883,178 France Mar. 15, 1943 OTHER REFERENCES Research Reports on Quartermaster Contract Projects from July 1, 1944, through June 30, 1945, and July 1, 1945, through October 31, 1945. Massachusetts Institute of Technology, Food Laboratory Reports. Pages 180, 181 Main Section. Section I, pages 2, 3, sheet S-6 and sheet 8-12.

A Method for the Production of Dry Powdered Orange Juice, by J. C. Sluder et al., Food Technology; vol. 1, No. 1, January 1947, pages 85 to 94. 

